Radar level gauge (RLG) systems are in wide use for determining the filling level of a product contained in a tank. Radar level gauging is generally performed either by means of non-contact measurement, whereby electromagnetic signals are radiated towards the product contained in the tank, or by means of contact measurement, often referred to as guided wave radar (GWR), whereby electromagnetic signals are guided towards and into the product by a waveguiding structure, such as a probe or a still pipe. The waveguiding structure is generally arranged to extend vertically from the top towards the bottom of the tank. The waveguiding structure may also be arranged in a measurement tube, a so-called chamber, that is connected to the outer wall of the tank and is in fluid connection with the inside of the tank.
The transmitted electromagnetic signals are reflected at the surface of the product, and the reflected signals are received by a receiver or transceiver comprised in the radar level gauge system. Based on the transmitted and reflected signals, the distance to the surface of the product can be determined.
More particularly, the distance to the surface of the product is generally determined based on the time between transmission of an electromagnetic signal and reception of the reflection thereof in the interface between the atmosphere in the tank and the product contained therein. In order to determine the actual filling level of the product, the distance from a reference position to the surface is determined based on the above-mentioned time (the so-called time-of-flight) and the propagation velocity of the electromagnetic signals.
Most radar level gauge systems on the market today are either so-called pulsed radar level gauge systems that determine the distance to the surface of the product contained in the tank based on the difference in time between transmission of a pulse and reception of its reflection at the surface of the product, or systems that determine the distance to the surface based on the phase difference between a transmitted frequency-modulated signal and its reflection at the surface caused by the time difference. The latter type of systems are generally referred to as being of the FMCW (Frequency Modulated Continuous Wave) type.
The propagation velocity, knowledge of which is required to determine the filling level based on the time-of-flight, is determined by various factors, such as the configuration of the waveguiding structure and environmental conditions inside the tank. Such environmental conditions, for example, include the composition of the atmosphere above the surface of the product contained in the tank.
For radar level gauge systems adapted for very high accuracy (such as 0.1% or less) the tank atmosphere will influence the propagation velocity. As an example, the propagation velocity in normal air is around 0.03% slower than in vacuum, while pressurized hydrocarbon gases have a higher influence with propane at 10 bar as one example of high influence (1%).
U.S. Pat. No. 6,867,729 and U.S. Pat. No. 5,249,463 disclose different systems designed to compensate for varying vapor concentrations in the atmosphere above the surface of the product in the tank.
The level measuring system disclosed in U.S. Pat. No. 6,867,729 normally operates at a relatively low gain to determine a material level of material contained in a tank, and periodically operates at a relatively high gain to determine a distance to a target marker provided along the probe above an expected sensing region of the probe. The determined distance to the target marker is used to compensate the determined material level for properties of vapor above the material level.
The level measuring system for measuring a water level disclosed in U.S. Pat. No. 5,249,463 comprises a probe provided with a pair of spaced reference discontinuities above the maximum level of the water. The difference between the measured and the known distance between the reference discontinuities is used to provide a measurement of the water level, that is independent of changes in the dielectric constant of the vapor above it.
Although enabling a determination of the propagation velocity in the tank atmosphere, reference reflectors such as those disclosed in U.S. Pat. No. 6,867,729 and U.S. Pat. No. 5,249,463 will create some disturbance in their vicinity with an influence on the measurement accuracy of the radar level gauge system. Typically, the calibration echoes are either too strong and disturb the filling level measurement, or too weak and are disturbed by the surface echo.